A. One-Way Wireless Transmission
There is a movement in the wireless industry towards providing more than simple numeric telephone number messages. These alphanumeric messages are typically originated from personal and office computers and sent to the wireless transmitting system via a telephone network. These messages are received by the messaging system controller (paging terminal) and processed for transmission via a radio transmitting system.
E-mail services have gained tremendous popularity and it is predicted that the current more than 17 million electronic mail (E-mail) subscribers will grow to 53 million by 1995. The average E-mail message is approximately 450 characters in length and 5 to 8 messages are sent each working day.
Personal computers have become far more compact in size permitting them to "move" with the person verses remaining in a fixed location. It is predicted that within the next few years, the majority of the personal computers will be less than 8 pounds in weight making them extremely convenient as a "portable office". This will make wireless communications a media of choice to accommodate portable office computers to receive information services and E-mail messages.
This places an extreme burden on the existing radio infrastructure that is allocated for messaging services. Currently, most metro area paging systems operating in the 150 and 450 MHz. radio bands are operating at or near full capacity accommodating current numeric paging subscribers. There is not adequate reserve air time available to accommodate alphanumeric information and E-mail services.
Nine hundred MHz. authorizations are currently available for local and regional paging implementation. However, at the current protocol speeds and the projected growth rates, the national channels will undoubtedly reach saturation within the next few years. Currently, one or more of the 900 MHz. nationwide paging channels are close to such a saturation. There is a pressing need to increase the air time efficiency of these radio paging systems.
Furthermore, U.S. Pat. Nos. 4,849,750, 4,851,830, 4,853,688, 4,857,915, 4,866,431, 4,868,562, 4,868,558, 4,868,860, 4,870,410, 4,875,039, 4,876,538, 4,878,051, 4,881,073, 4,928,100, 4,935,732, 4,978,944, 5,012,235, 5,039,984, 5,047,764, 5,045,850, 5,052,049, 5,077,834 and 5,121,115 disclose a frequency agile information transmission network and frequency agile data receivers. The above-referenced patents are incorporated herein by reference in their entirety.
U.S. patent application Ser. No. 07/702,939, now U.S. Pat. No. 5,436,960, filed May 20, 1991, entitled "Electronic Mail System with RF Communications to Mobile Processors"; U.S. Ser. No. 702,319, filed May 20, 1991, entitled "Electronic Mail System With RF Communications to Mobile Processors Originating From Outside of the Electronic Mail System" (now abandoned), U.S. Ser. No. 08/247,466, now U.S. Pat. No. 5,438,611 filed May 23, 1994, entitled "Electronic Mail System With RF Communications to Mobile Processors Originating From Outside of the Electronic Mail System and Method of Operation Thereof"; and U.S. Ser. No. 07/702,938, now U.S. Pat. No. 5,479,472, filed May 20, 1991, entitled "System for Interconnecting Electronic Mail Systems by RF Communications," disclose a system for linking an electronic mail system to portable computers using one-way wireless transmissions which may use the network and receivers disclosed in the aforementioned patents. These applications are incorporated herein by reference in their entirety.
Collectively, the above improvements utilizing the existing 150 and 450 MHz. radio messaging infrastructure will produce a significant reduction in the message delivery cost to the wireless subscriber. The cost to deliver a 450 character message with the system described in the above-referenced patents has been projected to be approximately 65.cent. versus $1.50 for a 50 character message the industry is currently offering subscribers. This significant cost reduction would further enhance the growth rate of the wireless information and E-mail service industry.
Furthermore, recently reallocated narrow band spectrum in the 220 MHz. radio messaging infrastructure is applicable to local and national data transmission for applications such as electronic mail. However, the narrow bandwidth of the channels in the 220 MHz. radio infrastructure does not support high data throughputs with prior art data protocols.
Adequate reserve radio spectrum is available in the 150 and 450 MHz. radio bands in the form of IMTS mobile channels that have been authorized for one- and two-way information transmission to transmit data and E-mail. However, a more reliable one-way messaging protocol is needed to accommodate the need for information and E-mail services. An additional requirement for a more air time efficient (faster) message protocol exists.
The POCSAG protocol was originally authored by the British Post office code Standardization Advisory Group. It was primarily developed for "tone only" or "semi-synchronous paging format". Unlike a synchronous paging format that must be transmitted continually to maintain synchronization of all the paging receivers, the POCSAG protocol is somewhat asynchronous in the respect that it only needs to send a radio signal when messages are about to be delivered. However, a POCSAG protocol transmission is extremely sensitive to atmospheric fades which are discussed below. If a three bit error exists in a transmission of information to a POCSAG protocol receiver, the BCH error correction code of the frames may be ineffective to prevent the transmission synchronism between the transmitter and receiver clock from being lost which results in a failure to complete the transmission of the information to the receiver and the receiver reverting into a scanning mode to attempt to lock onto a new transmission containing its identification code. A three bit error is produced by a fade in reception level below the detection level of the receiver for a time interval such as 2 to 4 milliseconds for 1200 and 512 baud data rates respectively.
To gain insight as to the POCSAG protocol, reference is made to FIG. 1 for the following explanation. A POCSAG protocol frame set consists of a PREAMBLE, a SYNC signal, and eight frames that are subdivided into two code words each. POCSAG protocol pagers are synchronous in the respect that once they detect the PREAMBLE and synchronize on the SYNC code word, they only then search for a message in their respective frame. If capcode ID numbers are consecutively assigned, the page is automatically assigned to a respective frame. Taking the binary equivalent of the last three digits of the ID of the pager, it is possible to determine in which one of the eight frames a respective pager would be located.
The POCSAG protocol pager is continually sampling the radio channel to look for its PREAMBLE. The PREAMBLE is typically 1.125 to 3 seconds in duration, and consists of an alternating string of ones and zeros sent digitally. When the pager samples the radio channel and determines the PREAMBLE string, it remains on and searches for the SYNC signal. The SYNC signal is actually a 62.5 millisecond code word that transmits a fictitious address to which the pagers respond. It is an unused address, and therefore does not cause falsing (erroneous turn on) of other pagers. Upon receiving the SYNC code word, the pager searches for a message in its respective frame group.
The POCSAG protocol has some inherent inefficiencies in its design. These inefficiencies exist in both 512 and 1200 baud POCSAG protocol pagers and are inherent in the architecture of the POCSAG protocol. In the POCSAG and other digital protocols, the baud rate is also the frequency of the subcarrier (e.g. 512 baud uses a 512 Hz. squarewave subcarrier which is modulated with ones and zeros to encode the parts of a transmission). Referring again to FIG. 1, it should be noted that a frame consists of code word one and code word two. If a POCSAG protocol pager receives a message, the first code word of its frame contains the ID or address information for the pager. It also contains alerting information to indicate to the pager what types of beeps are being issued. Code word two contains the numeric or alphanumeric information. When the pager is in numeric mode, the code word can contain five numeric digits.
However, it should be noted that very few numeric pages are five digits in duration. In fact, in a typical paging system, 98% of the numeric messages are seven digits in duration as illustrated in FIG. 2. Because the numeric message is seven digits in duration, the POCSAG protocol permits a borrowing or an extension into the next frame. The first code word in each frame (which would typically be an address), has a marker bit that indicates whether the code word contains address or numeric information. The remaining two digits of the seven digit example then spill over into the first code word of the next frame. Of the twenty bits of data (five numeric digits) in the next frame word, only eight would be used rendering the balance of the code word useless. The POCSAG protocol fills the balance of the code word with "filler code". The second code word of the adjacent frame is also back filled with filler code. The adjacent frame is unavailable for use by any other page. In fact, any message awaiting an adjacent frame two pager, must wait until the next frame group is sent in order to receive the message. The architecture of a POCSAG protocol based system requires that a message to a given pager be sent only in its respective frame.
It is obvious that unless. great care is utilized in the distribution of receivers to divide the receivers evenly within the frame groups, and that the customer usage in each frame group is equal, severe system air time inefficiencies are obtained. System air time efficiencies can vary between 30 and 60%. A great deal of the air time inefficiency cannot be fully utilized as it is due to the message length (seven digits) and is caused by the insertion of filler codes. If a per message comparison of paging protocols is made, it does not take into account the inherent system inefficiencies when numerous pages are sent. As mentioned previously, POCSAG protocol efficiencies vary considerably if a great deal of attention is not paid to the proper distribution of ID codes.
To gain some insight as to how the POCSAG protocol tends to reduce the air time efficiency, reference is made to FIG. 3. FIG. 3 shows fifty numeric pages that need to be sent via the paging system. For purposes of this example each page is a seven digit numeric page and the pagers are equally distributed between the eight frame groups.
The first problem is that due to each page being seven digits in duration, only an average of 3.5 pages can be sent per frame group. It should also be noted that a seven digit numeric page destined for the eighth frame group necessitates that a SYNC signal be sent. The message then spills over to the first frame of the next frame set. An "overhead" problem that also becomes obvious is that the receivers must be resynchronized after the transmission of the first eight frames. This resynchronization adds to the length of each message sent within the eight frame group. SYNC is 62.5 MS divided by 31/2 pages to apportion overhead. One hundred sixty seven milliseconds of the 267 MS period produces a 62.5% efficiency. Due to the spilling over of messages into their adjacent frames, it is seen that a second problem is precipitated. Assuming that each of the pages arrive in frame group order (e.g. 1234567, 1234567), it is seen that even if the paging terminal can sort to get the maximum 3.5 message per frame group efficiency, that a number of pages destined for the first frame tend to build or stack up. To eliminate this problem, fewer pagers may be issued in the eighth frame group (which spill over into the first frame group). However, the problem is not solved by doing so, and simply a build up of other pages in other frame groups occurs.
The 512 baud POCSAG protocol transmits 2.857 alphanumeric characters per 62.5 MS code word. If an alphanumeric message is transmitted in the first frame, a maximum of forty two characters can be sent before a 62.5 SYNC signal is required.
Maximum Characters Transmitted Frame Before Sync 1 42.8 2 37.14 3 31.43 4 25.71 5 19.99 6 14.89 7 8.57 8 2.85
As the national average length of an alphanumeric message is forty five characters, it is reasonable to add the SYNC overhead to the character time. The average E-mail message is considered to be from 150 to 450 characters, which increases the air-time requirements and increases the probability for a reception error.
 21.98 m.s. Per character 2.73 m.s. Frame OVHD per character 24.71 m.s.
Current digital protocols (POCSAG and Golay) are difficult to speed up due to their respective architectures. Attempts to increase POCSAG protocol speed from 512 to 1200 or 2400 baud (subcarrier frequency) have encountered the following problems.
The 1200 and 2400 baud data transmission rates have shortened the data bit time to approximately 800 and 400 microseconds respectively. This short time per bit produces a marked degradation in message receipt reliability for lengthy alphanumeric messages.
POCSAG protocol receivers have a BCH error correction scheme that can tolerate only one or two bits per frame to be erroneous before the transmitted character is unrecoverable. Man-made noise and Rayleigh fading phenomenon are very prevalent for such short bit times. The net result is that the cumulative effect of the error correction scheme that the current digital protocols utilize in combination with the effects of natural and man-made interferences degrade the message receiver's reliability when attempts are made to accommodate information and E-mail services. A three or more bit error represents a fade below the threshold detection level of the receiver which can cause the receiver clock to loose synchronism with the transmitted information, turn off and search for another transmission of its address. A three bit error represents a true message error which results in the loss of at least some data.
Speed per message is actually a relatively poor method to choose which format (type) of pager to utilize on a system. There are subtle differences in the various alpha signaling schemes that have far more impact on the reliability of the paging system and its ability to deliver message information to pagers. Differences in air time efficiencies and the techniques employed to correct erroneously received data by the pager are very important considerations that should be made.
The POCSAG and Golay protocols have digital formats requiring digital transmitters.
The 512 baud POCSAG protocol utilizes thirty one bit words, utilizing eleven of the bits for error correction. A three bit error in the address, as stated above, causes the message to be missed. This equates to a four millisecond fade or noise burst during the address and a two millisecond fade error during the message. Twelve hundred baud POCSAG protocol pagers have the same error correction format and air time inefficiencies. The fade resistance is reduced to a two millisecond fade during the address and one millisecond during the message. Although the number of pagers on a given channel is doubled, the degradation of message reliability due to the reduced fade resistance becomes noticeable with numeric paging, and markedly poor when long alphanumeric messages are sent.
The Golay protocol utilizes twenty three bit words, utilizing eleven of the bits for error correction. The Golay protocol transmits the ID code at 300 baud to increase the decoding reliability. The message is transmitted at 600 baud. The Golay protocol has an increased reliability for detecting the ID portion of the page due to the slower data rate. However, the overall signalling when the format is analyzed is noticeably slower than 512 baud POCSAG protocol, making it a poor choice to attempt to accommodate alphanumeric information and E-mail services on a radio channel that is currently accommodating tens of thousands of numeric pagers.
In the late 1980's A European consortium of countries formed a committee to develop a Pan-European wide paging network that would meet the requirements for the European traveling paging marketplace. Representatives from each country participated in a committee-like fashion to develop a new paging protocol that would allow the equivalent of an international paging network with common frequencies and a common protocol, permitting all countries to effectively offer Pan-European paging services. The European Radio Message Service (ERMES) committee was formed and developed both a new multi-level FSK paging format and all of the corresponding network architecture to relay messages to the transmitting infrastructure in Europe.
A multi-level FSK modulation technique is used with the ERMES protocol that modulates the transmitter at 3200 baud with each baud or FSK level representing two bits of binary information. The effective data rate of the ERMES protocol is therefore 6400 bits per second. The multiple baud level FSK modulation technique suffers from a reduced signal to noise ratio of six dB consequent from the lower level of the two level modulation being closer to the noise level of discrimination of the receiver. Loss of signal level for a substantial time below the noise level results in loss of synchronism which terminates reception of the remainder of the message resulting in a catastrophic message reception failure.
With a number of years of experience in utilizing the POCSAG protocol, the ERNES committee corrected some of the inherent deficiencies of the POCSAG protocol. By the same token, many parts of the architecture of the POCSAG protocol were utilized in the adoption of the ERMES protocol.
Somewhat later than the development of the ERMES protocol, a movement occurred in the United States to develop a more reliable radio messaging protocol. Although there are some manufacturers that have attempted to give proprietary names to this American protocol, it has typically been called the modified ERMES protocol. This is in part due to the fact that a large percentage of this modified ERMES protocol derived its architecture from the ERMES standards. Unlike the ERMES protocol, which is exclusively synchronous and transmits only at 3200 baud, the modified ERMES protocol has been proposed in three distinct phases. This is in part due to the fact that the American marketplace did not have a new band of frequencies allocated exclusively for national paging use. The modified ERMES protocol has to have the flexibility in its design to permit co-residing on currently operating radio messaging channels that contain POCSAG and Golay protocol pagers.
Phase one of the modified American ERMES protocol will utilize a 1600 baud FSK architecture that permits it to be compatible on existing digital base stations with other digital paging formats. Phase two, although not currently well defined, will transmit at 3200 baud with a 3200 bit per second rate utilizing multiple level FSK transmission. Phase three will utilize 3200 baud multi-level FSK modulation with each FSK level representing two bits of information for a 6400 bits per second transmission rate.
The 1600 baud modified ERMES protocol has been designed as a time-slotted, fully synchronous protocol. It derives much of its architecture from three previous signaling technologies. Message interleaving to increase fade resistance and the basic structure of the error correction and data blocks are derived directly from the European ERMES protocol. The time synchronization techniques are similar to previous RDS and MBS synchronous subcarrier systems that were developed in Europe for subcarrier messaging. The basic BCH error correction code and messaging architecture have been derived from the POCSAG protocol.
The 1600 baud modified ERMES protocol consists of 128 frames of information that are sent over a period of four minutes. A frame is composed of 150 MS of synchronization preamble and eleven blocks containing information each being 160 MS in duration.
The 150 MS of synchronization preamble contains three basic components called sync one, frame information, and sync two. The sync one portion permits the receiver to synchronize upon waking up during its respective frame. Frame information is then transmitted that can alert the receiver as to the proposed data rate that would be transmitted during the balance of the frame.
Sync two then permits the receiver to transition to the new baud rate if indeed the baud rate is different than 1600 baud.
The eleven blocks of information that follow the synchronization are typically divided into three categories. Blocks zero and one are typically utilized for addressing of the receivers contained within that frame. Approximately eight addresses can be contained per block permitting as many as sixteen receivers to be addressed in a single frame. Block two typically contains message vectors. A message vector points the receiver that was addressed in block zero or one to a following block to locate its messages. Blocks three thru ten will then contain messaging information for the receivers addressed in blocks zero and one.
Like in the POCSAG protocol that has been previously described, a BCH error correction code is utilized with the modified ERMES protocol with thirty-two bits per frame with eleven bits of error correction code to permit the receivers to correct bit errors up to a two bit error. Furthermore, the messages are interleaved so that all of the bits for a particular numeric or alphanumeric character are not transmitted sequentially. The low order bits of each character are transmitted first, followed by ascending order binary bits until the entire message has been sent. This increases the fade resistance of the 1600 baud protocol to approximately ten MS. Each block contains eight thirty-two bit words that then permit each word to contain five numeric digits or 2.85 seven bit alphanumeric characters. This portion of the 1600 baud protocol is identical to that of the POCSAG protocol and its architecture. Each block is therefore capable of containing as many as forty numeric digits of 22.8 seven bit alphanumeric characters.
Like the POCSAG protocol, certain restrictions apply to the 1600 baud modified ERMES protocol. Any portions of a block that are unused by message information must be filled with filler code. This introduces the same type of inefficiencies that are present in the POCSAG protocol.
As the eight words within a block have to contain five numeric digits and the bulk of numeric messaging requires seven digits, a significant percentage of filler code must be utilized to fill in the remainder of the unused words within a block.
A second inefficiency exists in the inherent architecture of the 1600 baud modified ERMES protocol that also exists in the POCSAG protocol. Pagers are selectively assigned to "time slots" and can only receive messages during their respective time slot, therefore great care in the even distribution of time slots (pager ID's) must be exercised. Like the POCSAG protocol, the 1600 baud modified ERMES protocol relies on the randomness of paging events to prevent excessive time delays being caused by multiple messages to pagers within the same time slot occurring at the same time. However, the 1600 baud modified ERMES protocol has a further encumbrance that its overall cycle time can be as long as four minutes. If the receiver's frame is unavailable due to high message traffic, it could wait as long as four, eight, or twelve minutes to receive its message.
This differs considerably from the POCSAG protocol in terms of time latency as the POCSAG protocol goes through a complete cycle in slightly over 1 second. If a POCSAG protocol frame for a respective pager is filled with other messages, the next frame will be available for transmission in one second.
To overcome this problem, the 1600 baud modified ERMES protocol has proposed utilization of less than the full one hundred twenty eight frames. However, this tends to have two factors that are detrimental to the paging subscriber. The first is a respective shortening of battery lifespan, and the second is a crowding of receivers into their respective frames. This crowding and clustering of receivers into smaller numbers of frames will tend to extend the waiting period by multiples of time that are dependent upon how quickly the frames cycle.
It has been proposed that in order to allow the 1600 baud modified ERMES protocol to be intermixed with current POCSAG protocol traffic, the frame duration should be shortened to one cluster of frames per minute. This basically introduces a sixty second time delay that, when averaged, would equal at least a thirty second message latency. However, during peak busy hour periods, this latency would be in multiples of not thirty seconds average, but thirty seconds for the first delay plus sixty seconds for each delay cycle thereafter. Simply explained this means that if a pager were to have to wait until the next frame, there would be an average delay of thirty seconds plus sixty seconds, or ninety seconds total. If the delay were to be two frames, it would be thirty seconds average plus sixty seconds average plus sixty seconds or two and one-half minutes.
Like the POCSAG protocol, the air-time latencies degrade the modified ERMES protocol considerably when addressing alphanumeric messaging. The national average for an alphanumeric message is forty characters. The 1600 baud modified ERMES protocol (like POCSAG) can borrow blocks of information to permit the forty character message to be delivered to a receiver. As each block has a maximum capacity of 22.8 characters, two blocks will be needed to transmit the alphanumeric message. If messages of longer duration are desired, the maximum character length for a single frame would be approximately one hundred eighty characters.
Like the POCSAG protocol, the 1600 baud modified ERMES protocol utilizes only seven bit characters. In order to address eight bit characters (as commonly used by computers), it is necessary to send commands that permit the receiver or external device connected to a receiver to permit the equivalent of straight binary information to be transmitted. This places a great deal of overhead on the external devices to receive this binary information and process it into true eight bit characters after decoding the interleaving and BCH error correction codes.
In terms of protocol efficiency, it appears that the 1600 baud modified ERMES protocol has slightly more overhead than the POCSAG protocol. Although the address information is similar to the POCSAG protocol, additional information containing message vectors. must also be transmitted to the 1600 baud receiver. As the block architecture and the BCH error correction code are identical, this would tend to lower the effective data throughput rate of the 1600 baud modified ERMES protocol.
A potential for interference exists when POCSAG and modified ERMES protocol pagers are interleaved on a channel. The POCSAG protocol typically transmits at 512 baud. The time per baud is 1953 microseconds. The 1600 baud modified ERMES protocol has a time duration of 625 microseconds per baud. Three bauds would equal 1875 microseconds. A comparison of the 1875 microsecond baud duration and the POCSAG protocol 1953 microsecond duration, yields less than a 5% time differential.
POCSAG protocol pagers, in order to quickly synchronize to the preamble, have a relatively wide synchronization bandwidth. For example, a 512 baud POCSAG protocol pager is capable of synchronizing to any data rate between 400 and 600 baud. This wide bandwidth is necessary to allow the POCSAG protocol pager to synchronize in a minimal amount of time to the POCSAG preamble. Although somewhat of a misnomer, the preamble is actually the portion of the POCSAG protocol signal that time synchronizes the receiver. The sync word that follows the preamble only serves to tell the pagers the correct bit timing order. The sync word is actually an unused ID or capcode that the POCSAG protocol pager searches for to obtain a match. Once a match is found, the POCSAG protocol pager can then establish the bit order or significance and can then begin proper decoding of the binary information that follows. It also uses the sync word to begin counting to permit it to decode a message in its corresponding frame.
As a POCSAG protocol pager detects the 1600 baud data rate transitions, it attempts to search for the synchronization or the sync code word. During the presence of 1600 baud information, the POCSAG protocol pager remains on for as long as several seconds after the completion of 1600 baud data transmission. As will be described later, this causes a severe degradation in the POCSAG protocol receiver's battery life when attempts at intermixing 512 and 1200 baud POCSAG protocol receivers with the 1600 baud modified ERMES protocol are made.
The 1600 baud data transmission of the modified ERMES protocol apparently has another adverse effect on the POCSAG protocol receiver. As a consequence of POCSAG protocol receivers relying on the preamble to determine their bit timing synchronization and having to maintain such synchronization for at least one second, another detrimental effect in the intermixing of the 1600 baud modified ERMES protocol with the POCSAG protocol is experienced. Once POCSAG protocol receivers synchronize to the 1600 bit per second data rate, they are not capable of re-syncing to the true POCSAG data rate if a POCSAG message immediately follows a 1600 baud message. To overcome this problem, one manufacturer has recommended that a POCSAG protocol warm-up be transmitted after a 1600 baud modified ERMES protocol message has terminated. This "POCSAG protocol warm-up" consists of 400 MS of 750 baud data of zeros and ones to be transmitted prior to the transmission of a 512 baud POCSAG protocol message. Although it has been termed a POCSAG protocol warm-up, it is quite to the contrary. Sending 750 baud to a POCSAG protocol pager will not cause the POCSAG protocol receiver to wake-up and attempt to synchronize. However, if the POCSAG protocol pager were on and synchronized to the 1600 baud modified ERMES protocol, the transmission of the POCSAG protocol warm-up will immediately cause the POCSAG protocol pager to return to the sample mode. Therefore, it appears that this 750 baud POCSAG protocol warm-up is instead a de-sync signal.
A common misconception in the wireless industry concerns the term "baud rate". It is easy to conceive that a higher baud rate directly controls the number of pagers per channel. This is in part due to the fact that baud rate as pertaining to computers is thought in "wireless" terms when a calculation as to the character speed is determined. Typically, a computer sends eight to eleven bits of information per character, and one simply divides that number into the baud rate to determine how fast information is transmitted. The fact is the baud rate is only a portion of the analysis. Unlike wireline computers that are connected with telephone lines, radio transmission requires additional "overhead" to be added to the signaling protocol due to its "one way" nature. Radio paging or one-way information transmission does not have the wire-line or two-way wireless privilege of requesting a second transmitted message if an error is received. Radio paging is a "one time" transmission that is "one way". Manufacturers of radio paging equipment therefore must encode additional information to permit the pager to correct errors caused by radio transmission problems. Instead of eight to eleven "bits" representing a character, as many as thirty bits may be required. This correction overhead is called "error correction code", and in some methods reduces the data transmission rate to the pager by as much as 75%. If half of the 1200 baud data rate is utilized for error correction, the effective data rate is 600 baud. The speed or "baud" rate is further reduced by "overhead" SYNC and "wake-up" preambles that must be sent to prepare the paging receiver prior to the transmission of an actual message.
Error correction code embedded in a frame of data bits is used to serially process the bits of the frame to correct minor bit errors such as one or two bits which occur anywhere in the frame. The serial processing of the bits of a frame which contain data and error correction code is typically implemented with a series of EXCLUSIVE OR gates. When a number of bit errors in a frame exceeds the error correction capacity of the error correction code, the data within the frame is erroneous. The prior art methods of wireless data transmission do not permit the recovery of valid data bits from a frame containing a number of bit errors which exceed the bit error correction capacity of the error correction code which for most types of data transmission protocols is two bits.
The operating environment has by far the greatest impact on the reliability of the paging system. Geographical terrain, the operating frequency, the presence of man-made structures, and natural and man-made noise all have a collective impact on the operating efficiency of the current state of art in paging systems. If the radio signal cannot reach the paging receiver, the sensitivity of the receiver or the error correction code in the protocol has little purpose. The first requirement of a paging system is to therefore provide a good radio paging signal at all the areas of the paging system's service area.
Geographical terrain of the paging service area determines the number of transmitter sites and the antenna patterns required to provide the necessary "Carey" coverage or service area. The less the variation in terrain, the more evenly distributed the RF field is, and the easier it is to obtain reliable service area coverage. Man-made objects (such as buildings) and geographic variations (hills) tend to cause shadows by blocking the "line of sight" paging signal. In a metropolitan radio environment, the receiver is subjected to a very hostile environment. The paging receiver is subject to multi-path interference, impulse noise, simulcast beats, and in many systems with multiple transmitters, non-synchronization of the transmitters. These phenomena are further compounded by building shadow effects and building penetration attenuation of the signal. All of the mentioned phenomena serve to reduce the reliability of the receiver. Higher power transmitters and multiple transmitters can alleviate a portion of the aforementioned problems, and increase other problems (e.g. multi-path, simulcast beats, and non-synchronized transmitters). It is not a simple problem to resolve, as numerous other problems exist which complicate the reliability of radio messaging services in a given area.
Previously, analog pagers utilized forms of active filters to decode the addressing tones. The active filters in the pagers were very sensitive to any form of phase or any other form of distortion that modified the sinusoidal signaling wave forms. Analog pagers required a "perfect" sine wave to properly decode and alert the user. Hence the reason for precise phasing of transmitters and synchronized transmitters (simulcast) systems were necessary to accommodate the active filter decoders in the pagers. Even with synchronization and, proper phasing of the station, the pager often decoded unreliably when located at the midpoint between two transmitters.
The move to digital encoding methods resulted from these former analog problems. In the early 1980's, digital transmitting and paging products were introduced by manufacturers that did not experience the problems associated with analog pagers. It was thought of as the only method to reliably send numeric data to a paging receiver. Considering that in 1980 analog technology was limited to 300 baud and yet digital technologies could transmit 600 to 1200 baud data, this was correct. It was not an inexpensive move, as literally every piece of equipment in the carrier's system required replacement. Paging terminals, base stations, and modems had to be purchased to replace the existing analog equipment. Digital paging also required that additional base stations be added to provide the increased signal strength necessary for reliable data stream reception by the pager. The deficiencies found in the analog technologies were eliminated by the move. Digital pagers do not share the problematic phase errors found in their analog counterparts. Research for new developments in analog technologies was abandoned by the pager manufacturers for several reasons. By emphasizing sales of digital systems, communication equipment manufacturers could increase sales of replacement base stations and paging terminals dramatically.
In this decade, advances in analog decoding technology have increased dramatically. Data transmission rates of 19,000 baud on ordinary telephone lines are common (as compared to 300 baud in 1980). Microprocessor assisted digital signal processors are available on a single chip with decoding sensitivities unheard of in 1980.
Even with the increased transmitter maintenance, the cumulative effects of mis-synchronization of the radio transmitters, Rayleigh fading, and man-made noise reduce the reliability of the current digital receivers noticeably. The overall fade tolerance at 2400 baud is less than one millisecond. A gap in the data transmission in excess of one millisecond causes the message receiver to terminate the receiving process.
There is a need in the art for a messaging protocol to be compatible on both analog and digital radio transmitting systems. The above-referenced patents disclose a protocol which is compatible with analog and digital transmitters. The protocol disclosed in these patents is approximately 99% reliable for the transmission of a 450 character message but is slower than the POCSAG protocol by a factor of approximately four. The protocol disclosed in the above-referenced patents like the POCSAG and GOLAY protocols transmits serial data frames with embedded error correction code. This protocol is immune to a fade duration of up to 100 milliseconds. The radiated power required to broadcast this protocol is approximately equal to that required for the POCSAG or Golay protocols.
The majority of the messaging radio transmitting systems (220 MHz. has less) have radio channels allocated that utilize 5 KHZ. transmitter deviation limits and transmitted audio bandwidths that are limited to 300 to 3000 Hz. The digital transmitters currently in service have modems that may limit the data rate to 1200 baud (1200 Hz. subcarrier). Compatibility with the current transmitter infrastructure with any new protocol is imperative to provide universal compatibility. The 1200 baud limitation is typically a constraint by the current design of the integrated modems which the digital base stations utilize. The bandwidth of current radio transmitters can accept faster data rates if the bandwidth of the digital modems is increased.
As is apparent from the description of the POCSAG protocol above, these are fundamental problems of increasing its data throughput. The problems are caused by the propensity of atmospheric serial information transmission to semi-synchronous receivers to be subject to unpredictable interruptions caused by atmospheric fades which degrade the atmospheric transmission below the noise threshold of the receiver. As has been pointed out above, a three bit error may cause a total loss of synchronism between an information transmission and a POCSAG protocol receiver from which the receiver cannot recover with the remainder of the transmission after the fade being lost with the receiver going into a search mode to look for another transmission of an address of the receiver.
When the probability of a loss of synchronism becomes high, the use of a transmission medium goes down. The POCSAG protocol has a reliability of around 95% for a seven character message which means that a 5% chance exists of losing one or more digits of the transmission. A higher reliability is needed for data transmissions between computers to make one-way atmospheric data transmission a widespread methodology.
An analysis of atmospheric transmission using the prior art protocols in accordance with accepted mathematical relationships for evaluating atmospheric radio frequency transmissions follows which reveals that they are poorly suited to data transmissions of more than a few characters in length.
 Fading Rate (1) F.sub.o = SF/670 S = Speed MPH F = Frequency in MHz. F.sub.o = Hz Fade Length (2) t = 1/2rF.sub.o (e.sup.+.693r.sup..sup.2 -1) r = ST/SM Threshold/Median
The threshold ST is the receiver threshold detection level and the median SM is the median field strength level.
 Fade Below Threshold (3) F.sub.R = 2re.sup.-.693r.sup..sup.2 F.sub.o Probability of Message Loss (4) P(error) = 1 - e.sup.-F.sup..sub.R .sup.LP.sup..sub.w L = Message Time (Length) P.sub.w = Probability of fade larger than catastrophic failure length P.sub.w = 1.5e.sup.-1.1 t/t
The quantity t is the net probability of a fade divided by the mean rate of fading and equals EQU 1/2rF.sub.o (e.sup.+0.693r.sup..sup.2 -1) (5)
The fading rate F.sub.o is the natural frequency at which atmospheric radio frequency transmissions periodically fade as a function of the channel frequency F.sub.o and the speed of the receiver or transceiver if the system is a one-way or two-way wireless system in miles per hour; the fade length t in seconds is the length of fade; the fade below threshold F.sub.R is the time duration in seconds that a transmission drops below the detection capability of the receiving circuitry; and the probability of message loss Pack is the probability that a message transmission will not be completed as a result of a lost of synchronism between the data transmission and the receiver. See S. O. Rice; Statistical Properties of a Sine Wave Plus Random Noise; Bell System Technical Journal, January, 1948; T. A. Freeburg; An Accurate Simulation of Multipath Fading; Paper;1980; Caples, Massad, Minor; UHF Channel Simulator for Digital Mobile Radio; IEEE VT-29; May 1980; and P. Mabey, D. Ball; Application of CCIR Radio Paging Code No. 1; 35th IEEE V. T. Conf.; May 1985 for a discussion of the above-referenced equations.
FIGS. 4A-4J illustrate an analysis of the POCSAG protocol at baud rates of 512, 1200 and 2400 which are the currently used or prospectively to be used baud rates for frequencies of 150, 450, 900, 1200 and 2200 MHz. as a function of the velocity of the receiver in miles per hour. Specifically, FIG. 4A is for a numeric 7 digit POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 90 microvolt/meter (39 db) median electric field strength; FIG. 4B is for a numeric 7 digit POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 130 microvolt/meter (43 db) median electric field strength; FIG. 4C is for a 50 character POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 90 microvolt/meter (39 db) median electric field strength; FIG. 4D is for a 50 character POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 130 microvolt/meter (43 db) median electric field strength; FIG. 4E is for a 80 character POCSAG message with an 50 microvolt (18 db) detection sensitivity with a 90 microvolt/meter (39 db) median electric field strength; FIG. 4F is for a 80 character POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 130 microvolt/meter (43 db) median electric field strength; FIG. 4G is for a 200 character POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 90 microvolt/meter (39 db) median electric field strength; FIG. 4H is for a 200 character POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 90 microvolt/meter (43 db) median electric field strength; FIG. 41 is for a 450 character POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 90 microvolt/meter (39 db) median electric field strength; and FIG. 4J is for a 450 character POCSAG message with an 8 microvolt (18 db) detection sensitivity with a 130 microvolt/meter (43 db) median electric field strength. The Probability of Message Loss stands for the probability during an atmospheric message transmission that synchronism between the atmospherically broadcast message and the receiver will be lost during the message transmission causing the receiver (or transceiver if in a two-way wireless system) to revert to a search for the broadcast of the receiver's address marking a new transmission to the receiver. This equates to an error such as a 3-bit error with the BCH error correction code currently in use with POCSAG protocol. As is apparent, the performance of POCSAG seriously degrades as the message length increases. For example, a comparison of FIGS. 4A-4J reveals a significant increase in the probability approaching 100% that the reception of a message will not be completed as the length of a message reaches 450 characters. These error rates of message loss are unacceptably high for transmission to computers for applications such as E-mail. The prior art retransmission of messages at a later time after completion of the original message transmission does not significantly increase the probability of receipt of a message with each subsequent transmission only halving the probability of a successful transmission.
FIG. 5 graphically illustrates data from the table of FIGS. 4A-4J. As is apparent, the error rate approaches 100% as the message length increases. A whole family of similar curves may be plotted from FIGS. 4A-4J which reveal similar relationships of how the length of the message increases the probability of a 3-bit or larger error which correlates to a message failure.
FIG. 6 illustrates a diagram of a prior art encoding mechanism used to encode prior art paging protocols such as POCSAG, GOLAY, 2 Tone and 5/6 Tone, etc. This encoding mechanism has also been used to encode and decode two-way mobile data formats. This encoding mechanism is a Hi-Cap Multiswitch Model DMF-4000 manufactured by ESA Telecom Systems Group, Inc. of 10345 S. Oxford, Chicago Ridge, Ill. 60415. The encoding contains the necessary microelectronics to encode the protocols and forward them to the transmitter. The encoding mechanism utilizes a distributed processing architecture to permit the receipt of messages from the public switch telephone network (PSTN), provides the necessary subscriber verification and validation, encodes the protocols, and gain access to the radio transmitting system. The higher level processor consists of a central processing unit 30, a read only memory 32 that contains the BIOS, a random access memory 34 that stores in buffers both message and system operational information, a hard and soft disk drive 36 that are utilized to store the main operating program and subscriber file information, a printer/billing port 38 for the logging of system activity and service updates, maintenance port modem 40 for diagnostics in the event of a system malfunction, and a resident keyboard and monitor 42 to allow access to the main processing unit for addition of subscribers and system maintenance.
The main processor, which is comprised of items 30-42, contains the system operating program and control mechanisms that communicate to the peripheral modules 46-56 via the PCM matrix switch and data board buffers 44 and bus 58. The PCM matrix switch 44 contains the digital and audio matrix that permits any of the resident modules 46-50 to send audio and digital information between each other and the main CPU. It is also responsible for buffering data from and to the various peripheral modules 46-56 to permit the system to grow in size to accommodate the messaging traffic as needed. Each of the peripheral cards 46-56 contains one or more board resident processors that further process information and relieve processing overhead from the main CPU. It is with this distributed processing architecture which permits the encoding mechanism to be expanded to accommodate several hundred input ports and numerous radio channels. Dotted line bidirectional arrows are used to identify the two-way communication paths. Additionally, the radio station control may be comprised of multiple modules which each are connected to one or more base stations (not illustrated).
In order to gain a complete understanding of how the encoding mechanism of FIG. 6 functions, it is advantageous to understand how a message is processed from receipt by the encoding mechanism from the PSTN and ultimately delivered to the radio transmitting system connected to the encoding mechanism for transmission to the receiver. To send a message to the receiver, a message originator calls via the public switched telephone network PSTN to one of the encoding mechanisms telephone ports. Three telephone port configurations are described here being direct inward dial trunks 46, direct outward dial trunks 48, and/or mixed frequency trunks 50 which can both answer and originate calls. The three basic trunk configurations are necessary to accommodate the various telephone interfacing requirements that are necessary to interface from the PSTN to the encoding mechanism at its particular location. Details of the trunk configurations are known. The modems function to convert digital formatted information to analog for transmission by telephone lines. The protocol encoder 54 permits multiple protocols to be encoded which is common with paging systems which sequentially broadcast in different protocols. The radio station control interfaces the encoding mechanism with a radio transmitter or radio system control. If the protocol encoder 54 is encoding two-way protocols, one or more radio station controls 56 and/or one or more bidirectional lines are connected to the one or more radio station controls.
Upon receipt of the message recipient's telephone or ID number, the message entry process begins. The main CPU 30 looks up in the customer file the necessary message decoders that must be connected to the previously described telephone trunk modules. Referring to FIG. 4, the main CPU 30 may connect any number of modems individually or simultaneously to permit the decoding of medium to high speed serial data decoders. This is accomplished by connections through the PCM matrix switch 44 to one or more modem modules 52 that are connected to the digital data and PCM bus highways 58. In some cases it may not be known which type of entry modem or entry protocol is being. used, and in this situation the resident decoders on the respective telephone trunk modules 46-50 are responsible for decoding DTMF entry protocols and higher speed modem protocols are decoded by the modem modules 52.
The encoding mechanism of FIG. 6 is designed to receive numerous numeric and alphanumeric entry formats from the message originator. They include DTMF (Dual Tone Multiple Frequency) overdial for a numeric message that can be directly encoded from a telephone keypad. An alphanumeric DTMF entry process can be entered by a two button press entry scenario that corresponds to the desired alphanumeric character that is displayed on the keypad. Message originators that are utilizing a PC that have a modem can also enter a similar DTMF alphanumeric format by software packages that reside in the PC that direct the PC's modem to send DTMF tones. All of the aforementioned DTMF message entry formats are decoded by resident DTMF decoders on the respective telephone trunk modules.
Higher speed formats utilizing Bell and CCITT formats permit messages to be sent at 300, 600, 1200 and 2400 baud formats. In the event that the higher speed protocols are utilized, a modem module 52 is connected to the respective telephone trunk module via the digital data and PCM data bus 58. The modem module 52 is capable of auto-adjusting to the desired speed and format of the message originators modem.
Upon completion of receipt of the message, the main processor 30 is alerted to permit a message transfer. In the event of a DTMF message, the message has been temporarily stored on the respective telephone trunk module, or in the event of a higher speed data message it is stored and temporarily buffered on the modem module 52. The message is then transferred to the main CPU 30 for further processing via the data bus buffer module 44. The main CPU 30 then looks up in the customer file the format of the receiver and stores the message in the respective batch buffers for that particular encoding format. The encoding mechanism described is capable of encoding numerous signalling formats that include analog 2-tone, 5/6 tone, POCSAG and Golay protocols.
In order to optimize and obtain the maximum air-time efficiency, messages for receivers with like signalling protocols are buffered and batched and are controlled by two entries that are programmable via the systems menu. The two entries are time and volume related. The number of characters that can be transmitted when the system controller gains access to the radio transmitting system are programmable as well as a predetermined period of time and/or both. In the event of very low traffic periods, it is typically the time entry that will precipitate the transmission of the messages that are stored in the main processor's batching buffers. In the event of high activity, it is the volume or number of characters that trigger the main CPU 30 to initiate accessing the radio transmitting system.